This disclosure pertains to patches for repairing damage to structures, namely, self-stressing patches that use a combination of shape memory alloys (SMAs) and fiber reinforced polymer (FRP) composites.
Fatigue cracks form in steel structures due to cyclic loads such as wind, traffic, or machinery vibration. Under repeated loading cracks form and propagate which may compromise the integrity of the structure. Fatigue cracks initiate at locations of stress concentration such as notches, welded details or holes. Traditional methods of repairing cracked steel structures include bolting steel cover plates, drilling crack-stop holes (crack blunting), peening, or repair welding. The basic principal behind these approaches is to provide an alternate load path for stresses to bypass the crack tip, reduce the magnitude of stress concentrations near the crack tip, and/or to provide a residual compressive stress field near the crack tip to slow or halt crack propagation. However, these techniques require the use of heavy equipment and are often prone to crack re-initiation. Further, welding may be prohibited in some applications where there is a risk of explosion. Furthermore, all of these techniques require permanent, irreversible modification of the underlying structure.
Patching cracked steel members with fiber reinforced polymer (FRP) materials, is emerging as an effective alternative to repair cracked steel structures. Externally bonded FRP patches do not require welding or bolting, can be applied without permanently modifying the parent structure, do not corrode, and add little dead load to the repaired member. Using this technique, the FRP patch is bonded across the crack with a structural adhesive. The FRP bridges the crack and reduces the stress range near the crack tip. The effectiveness of this repair method can be improved by prestressing the FRP patch thereby providing compressive stresses near the crack tip. Research has shown that using prestressed CFRP patches can completely halt crack propagation and extend the fatigue life of cracked steel members. However, the prestressing operation typically requires hydraulic jacks, pumps, and complex fixtures which can limit the applicability of this method in many practical applications. Some alternative approaches have been proposed which involve the use of specialized fixtures and threaded rods to achieve the prestressing force. However, these techniques are generally designed for global prestressing of an entire member rather than localized patching at a discrete crack location.
What is needed, therefore, is an improved device or method for applying prestressing forces to a FRP patch.